Waste heat recovery system

ABSTRACT

A waste heat recovery system in which hot waste fluids, such as flue gasses, pass through a fluid heat exchanger configured to transfer energy in the form of heat to a heat transfer liquid, preferably molten salt. The energy in the molten salt is used to generate useable power such as electrical energy. The waste gas heat recovery system is especially adapted for use with batch processes, such as steelmaking and copper converting, and allows continuous or substantially continuous power production.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on, claims priority to and benefit of U.S. Provisional Application Ser. No. 61/181,284 filed on May 26, 2009, the entire contents of which are incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO APPENDIX

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The inventions disclosed and taught herein relate generally to processes that generate high temperature waste fluid stream; and more specifically relate to waste gas heat recovery from electric arc furnace (EAF) steelmaking processes using direct evacuation control (DEC).

2. Description of the Related Art

Many industrial processes, such as steelmaking, consume vast quantities of energy and produce a considerable amount of heat in the form of waste fluid, such as waste gas, that is typically unrecovered or wasted. The inventions disclosed and taught herein are directed to an improved system for waste fluid heat recovery and utilization from processes and in particular waste gas heat recovery from flue gasses in electric arc furnace processes using a direct evacuation control method.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention comprises a waste heat recovery system, comprising a source of fluid waste heat; a first heat exchanger configured to transfer energy from the hot fluid to a heat transfer fluid; a second heat exchanger configured to transfer energy from the heat transfer fluid to water to produce steam; and an electrical power generating system configured to convert the energy in the steam into electrical energy.

Other aspects of the present invention include an improved waste heat recovery system in which hot exhaust gasses, such as flue gasses, pass through a gas-to-fluid heat exchanger configured to transfer energy in the form of heat to a heat transfer liquid, preferably molten salt. The energy in the molten salt is used to generate useable power such as electrical energy. The waste gas heat recovery system may be adapted for use with batch operations that generate high temperature waste gas, such as steel making processes, and allows continuous or substantially continuous electric power production.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a prior art steelmaking process.

FIG. 2 illustrates a particular embodiment of the current invention in a steelmaking process.

DETAILED DESCRIPTION

The Figures described above and the written description of specific structures and functions below are not presented to limit the scope of what Applicants have invented or the scope of the appended claims. Rather, the Figures and written description are provided to teach any person skilled in the art to make and use the inventions for which patent protection is sought. Those skilled in the art will appreciate that not all features of a commercial embodiment of the inventions are described or shown for the sake of clarity and understanding. Persons of skill in this art will also appreciate that the development of an actual commercial embodiment incorporating aspects of the present inventions will require numerous implementation-specific decisions to achieve the developer's ultimate goal for the commercial embodiment. Such implementation-specific decisions may include, and likely are not limited to, compliance with system-related, business-related, government-related and other constraints, which may vary by specific implementation, location and from time to time. While a developer's efforts might be complex and time-consuming in an absolute sense, such efforts would be, nevertheless, a routine undertaking for those of skill in this art having benefit of this disclosure. It must be understood that the inventions disclosed and taught herein are susceptible to numerous and various modifications and alternative forms. Lastly, the use of a singular term, such as, but not limited to, “a,” is not intended as limiting of the number of items. Also, the use of relational terms, such as, but not limited to, “top,” “bottom,” “left,” “right,” “upper,” “lower,” “down,” “up,” “side,” and the like are used in the written description for clarity in specific reference to the Figures and are not intended to limit the scope of the invention or the appended claims.

Applicants have created an improved waste heat recovery system in which hot fluids, such as flue gasses, pass through a heat exchanger configured to transfer energy in the form of heat to a heat transfer liquid, preferably molten salt. The energy in the molten salt is used to generate useable power such as electrical energy. For example, a gas-to-fluid heat exchanger may be used to transfer heat from a waste gas to a heat transfer liquid, such as molten salt. The heat transfer liquid is preferable contained in a closed loop system that allows the heated fluid to be accumulated or stored for use as desired or needed. The heated fluid can be drawn from the storage facility to create a working fluid, such as steam, which can be used to generate electricity in a conventional steam turbine generator system. In the event the waste gas heat source is intermittent (e.g., batch) or goes off-line, the present invention is still capable of producing steam and electricity. The waste heat recovery system described herein is especially adapted for use with batch processes that generate high temperature waste gas streams, such as steelmaking processes, and allows continuous or substantially continuous power production.

Turning now to a description of current steelmaking technology, FIG. 1 illustrates an electric arc furnace (EAF) steelmaking process using direct evacuation control (DEC) of flue gasses during peak conditions. This conventional process moves from left to right and, for purposes of this disclosure, starts at the electric arc furnace 100. Flue gasses 110 exit the furnace at approximately 2,800-3,500° F. and at about 15,000 to 50,000 scfm. Combustion air 112 is introduced, either through an air gap 114 or by ducting into the off-gas and the resulting combined off-gas volume of about 50,000 to about 150,000 scfm is ducted through a water cooled duct 114 and then through a dropout chamber 120 to collect particulate matter. By the time the gasses exit the drop out chamber 120, the gas temperature has cooled to about 2,000-2,800° F. (1,093° C.-1,538° C.). At that point, it is conventional for the gasses to pass through a primary water cooled duct 130 to reduce the gas temperatures even further. Conventional water cooled ducts are typically between 8-12 feet in diameter and usually about 150-300 feet in length.

It will be understood by those of skill in the art that the water cooled ducts of FIG. 1 are fed by a cooling water tower 140 or other cooling water supply operating at about 10,000-30,000 gpm of cooling water at about 80-100° F. (26° C.-38° C.). After the flue gasses 110 from the furnace 100 pass through the water cooled duct 130, they typically are further cooled, conditioned, and then directed to a baghouse filtration system 150 or other gas treating process. Typically, the gasses leave the water cooled duct 130 at about 1,000-1,600° F. (538° C.-871° C.) and the water exits the duct 130 at about 100-120° F. (38° C.-49° C.) and at about 10,000-30,000 gpm. It will be appreciated that the energy contained in the flue gasses 110 of a conventional EAF steelmaking process is essentially waste heat that is rejected to the environment and not utilized for productive purposes.

Turning now to FIG. 2, one of many possible embodiments of the current invention is illustrated. FIG. 2 shows an improved method and apparatus for heat recovery during a steelmaking process. The particular embodiment is illustrated for an EAF process using a DEC system as discussed above. The exhaust gasses 210 are expected to exit the steelmaking furnace 200 at a rate of about 15,000-50,000 scfm. After combustion air 212 is introduced, either by a combustion air gap or by ducting, a resulting combined off-gas volume of about 50,000 to about 150,000 scfm will enter the first portion of the embodiment of the present invention.

At this point, the embodiment of the present invention illustrated in FIG. 2 recovers some of the waste heat in the exhaust flue gasses 210 by virtue of an air (flue gasses) to fluid (e.g., molten salt) heat exchanger 220. The flue gasses are ducted through a portion 222 of the fluid heat exchanger 220 and then through a dropout chamber to collect particulate matter. At that point, the flue gasses 210 continue to pass through a primary portion 224 of the fluid heat exchanger 220. The heat exchanger 222 is configured to accept the flue gasses after a combustion air gap and to transfer at least a portion of the energy therein to a heat transfer liquid. In this particular embodiment, it is preferred that the heat transfer liquid be a molten salt.

The heat exchanger 220 may be a molten salt cooled duct configured to transfer between about 30 and 40 megawatts-thermal (MW_(t)) to the molten salt, thereby reducing the temperature of the gasses to an expected amount of about 1,000-1,600° F. (538° C.-871° C.) and at about the same rate of 50,000-150,000 scfm. Preferably, the heat exchanger 220 will be made of high heat resistant materials, such as nickel-based alloys. For retrofit or upgrade installations, a molten salt cooled duct system 220 may effectively replace a conventional water cooled duct, e.g. 130 and 116. For example, and without limitation, a duct 220 maybe typically between 8-12 feet in diameter and 150-300 feet in length.

After the exhaust gasses 210 from the furnace 200 give up a portion of its energy in the gas-to-fluid heat exchanger 220, the gasses may be further cooled, conditioned, and directed to a conventional baghouse 230 or other treatment process.

FIG. 2 also illustrates a molten salt system 290 in which the duct 220 is fed by a “cold” salt tank 240. The salt 292 in the tank 240 is molten and, depending on the salt actually used, is expected to be preferably maintained at or about 550° F. (288° C.). Other temperatures may be desired. The “cold” molten salt 292 is pumped (not shown) into the duct 220 where it travels the length of the duct 220 absorbing heat from the flue gasses 210. FIG. 2 illustrates the flow of salt 292 and exhaust gas 210 to be counter current (in opposite direction), but it will be appreciated that the salt 292 could flow through the duct 220 in the same direction as the exhaust gasses 210. In a preferred embodiment, the duct 220 is expected to be configured to accept about 600-800 gpm of molten salt. As the molten salt 292 exits the duct, its temperature is expected to increase to about 950° F. (510° C.) which would represent about 30-40 MW_(t) transferred to the molten salt.

Molten salt 292 exiting the duct 220 may be routed to a hot salt tank 250. The hot salt tank 250 is configured to take advantage of the thermal properties of molten salt and may function as a thermal accumulator, as described below in more detail. Molten salt from the tank 250 (and/or directly from the duct 220) may be provided, e.g. pumped, to a power production system 260 to convert the energy transferred from the hot off-gases 210 to the molten salt 292 into useable power. For example, and without limitation, the power production system 260 may be any power production system, such as an electric power generating system. For example, energy from the molten salt 292 can be transferred through a fluid-to-fluid heat exchanger 270 to generate steam for a steam turbine generator 280. Such power production systems include steam condenser 262 and pump 264. The expected output of such a steam turbine system is about 8-12 about MW_(e). Of course, the output will vary based on the heat input and the power system used.

As shown in FIG. 2, the molten salt exits the power production system 260 and returns to the cold tank 240 to begin the cycle again.

It will be appreciated that because EAF processes are typically batch processes rather than continuous processes, the waste heat (i.e., flue gasses 210) available for the power production system 260, may be intermittent and/or discontinuous. Storage tank 250 can be configured to minimize or eliminate disruptions in the energy transferred to the power production system 260 from the molten salt. In a preferred embodiment, tank 250 holds enough molten salt 292 to provide up to one hour of power production system's 260 energy requirements.

Other and further embodiments utilizing one or more aspects of the inventions described above can be devised without departing from the spirit of Applicants' invention. For example, while the present invention has been described with respect to implementation in an EAF steelmaking processes, other industrial processes, such as copper converters, cement kilns, or other continuous or batch processes generating high temperature waste gas streams may benefit from this invention. Further, the various methods and embodiments can be included in combination with each other to produce variations of the disclosed methods and embodiments. Discussion of singular elements can include plural elements and vice-versa.

The order of steps can occur in a variety of sequences unless otherwise specifically limited. The various steps described herein can be combined with other steps, interlineated with the stated steps, and/or split into multiple steps. Similarly, elements have been described functionally and can be embodied as separate components or can be combined into components having multiple functions.

The inventions have been described in the context of preferred and other embodiments and not every embodiment of the invention has been described. Obvious modifications and alterations to the described embodiments are available to those of ordinary skill in the art. The disclosed and undisclosed embodiments are not intended to limit or restrict the scope or applicability of the invention conceived of by the Applicants, but rather, in conformity with the patent laws, Applicants intend to fully protect all such modifications and improvements that come within the scope or range of equivalent of the following claims. 

1. A waste gas heat recovery system, comprising: a source of waste heat fluid; a first heat exchanger configured to transfer energy from the waste heat fluid to a heat transfer fluid; a second heat exchanger configured to transfer energy from the heat transfer fluid to a working fluid; and a power production system configured to convert the transferred energy in the working fluid into electrical energy.
 2. The system of claim 1, wherein the working fluid is steam and the power production system further comprises: a steam generator; a steam turbine; and an electrical generator.
 3. The system of claim 1, wherein the source of the waste heat is flue gasses from a steelmaking process.
 4. The system of claim 3, wherein the steelmaking process comprises an electric arc furnace
 5. The system of claim 1, wherein the heat transfer fluid is molten salt.
 6. The system of claim 5 further comprising: a cold salt tank; and a hot salt tank, all arranged in fluid communication with the first heat exchanger.
 7. A waste gas heat recovery system, comprising: a source of waste heat gas; a first heat exchanger configured to transfer energy from the waste heat gas to a heat transfer fluid; a second heat exchanger configured to transfer energy from the heat transfer fluid to water to produce steam, or other vaporized fluid; and an electrical power generating system configured to convert the energy in the steam or vapor into electrical energy.
 8. The system of claim 1, wherein the source of waste heat gas is a steelmaking process.
 9. The system of claim 2, wherein the heat transfer fluid is molten salt.
 10. The system of claim 3, wherein the power generating system is a steam turbine.
 11. A method of recovering waste heat comprising: providing waste heat; passing the waste heat through a first heat exchanger configured to transfer energy from the waste heat to a heat transfer fluid; passing the heat transfer fluid through a second heat exchanger within a power production system.
 12. The method of claim 11 wherein the waste heat is provided by flue gasses generate by a steelmaking process.
 13. The method of claim 12 wherein the steelmaking process comprises an electric arc furnace.
 14. The method of claim 11 wherein the heat transfer fluid is molten salt.
 15. The method of claim 14 further comprising: providing the molten salt to the first heat exchanger from a cold salt storage tank; storing the molten salt in a thermal storage prior to passing the heat transfer fluid through a second heat exchanger within a power production system.
 16. The method of claim 11 further comprising generating steam in the second heat exchanger.
 17. The method of claim 16 further comprising the providing the steam generated by the second heat exchanger to a steam turbine.
 18. The method of claim 17 of generating electrical power from the steam turbine.
 19. The method of claim 14 further comprising generating steam in the second heat exchanger.
 20. The method of claim 19 further comprising the providing the steam generated by the second heat exchanger to a steam turbine. 